1. Technical Field
The invention relates to a rubber composition used in a sidewall and/or rim strip of a. tire, and a pneumatic tire using the rubber composition.
2. Related Art
A rubber composition forming a sidewall part or rim strip part of a pneumatic tire contains a wax or an age register in order to suppress deterioration by ozone or ultraviolet rays in the air. Those have a deterioration suppressing effect, but become a factor of poor appearance of a tire by whitening or brown discoloration of a rubber surface.
To suppress whitening of a sidewall part by a wax, JP-A-2014-210830 proposes to add a fatty acid metal salt having the carbon number of from 16 to 20 smaller than the carbon number of a hydrocarbon contained in the most in a wax together with a wax. JP-A-2014-015535 proposes to use a tacky wax comprising an ester of fatty acid having a hydroxyl group in place of a hydrocarbon type wax. However, it was not known that discoloration of a rubber surface can be suppressed by using both a hydrocarbon type wax having a specific melting point and a fatty acid processing aid having a specific melting point.
JP-A-2008-150435 and JP-A-2005-206673 disclose to add a fatty acid processing aid such as a fatty acid metal salt or a fatty acid amide to a rubber composition for a tire together with a hydrocarbon type wax. However, those documents are mainly the technology for establishing both tire performance such as low fuel consumption and processability in a rubber composition for a tread. Those documents do not disclose a combined use of a hydrocarbon type wax having a specific melting point and a fatty acid processing aid having a specific melting point.